Library API Improvements

[nloding]

Problem

Several library API improvements could enhance usability:

1. Batch operations: Add multiple games efficiently

   db.add_games(&[game1, game2, game3])?;

2. Async support: For large database operations

   async fn get_game(&mut self, game_num: u32) -> Result<Game>;

3. Streaming PGN writer: Avoid building entire output in memory

   fn write_pgn_to<W: Write>(&self, writer: &mut W) -> Result<()>;

4. Game validation: Legal move verification before encoding

   game.validate()?; // Check moves are legal

Priority

Low - future enhancements